The present invention relates generally to the design and fabrication of an x-ray imager, and more particularly to the design and fabrication of an x-ray imager in which the sensor arrays are protected from the corrosive effects of the scintillating material.
A conventional x-ray imager (10), as shown in FIG. 1, typically includes a barrier layer (30) between a photosensor array (20) and a scintillating material (40).
The scintillating material (40), such as cesium iodide doped with thallium, emits optical photons in response to absorption of incident x-ray radiation. Cesium iodide, in particular, is ideally suited as a scintillating material because, when deposited, forms needle-like columns (42) that provide light piping. The columns are typically several microns in diameter and several hundred microns long.
A conventional sensor array (20) is also shown in FIG. 1. The sensor array absorbs the optical photons emitted by the scintillating material and generates electrons in proportion to the energy flux of the photons. Photodiodes collect the charge which are periodically measured and then reset to a known charge in order to process electrical signals generated by the photodiode in response to the incident radiation.
The sensor array (20) is formed of a plurality of photosensitive elements arranged in rows and columns on a substrate (11). A conventional photosensitive element is shown in FIG. 2. Each photosensitive element (21) comprises a photodiode comprising a photosensitive island (24) of amorphous silicon (a-Si) over a bottom contact pad (26) and with an upper conductive layer (28) of indium tin oxide over the assembly. A passivation layer (50) is disposed under the upper conductive layer (28) except where the upper conductive layer (28) is in electrical contact with the upper surface of the of the photosensor island (24). The photosensitive element further includes a n-type doped region (23) and a p-type doped region (25). A conventional passivation layer (50) comprises a silicon nitride layer (27) and a polyimide layer (29).
The barrier layer (30) is typically silicon nitride, silicon oxide, or silicon oxynitride. It serves several purposes including protecting the sensor array from the scintillating material, providing a surface to which the scintillating material can adhere, providing optical coupling between the scintillating material and the sensor array, and protecting the sensor array from moisture. Performance of the sensor array can also be degraded by a number of factors including exposure to the solvents used in fabrication and exposure to the high temperatures used to anneal the scintillating material.
Kwasnick et al. (U.S. Pat. No. 5,401,668) disclose a two layer barrier layer to protect the sensor array from moisture and improve adhesion of the scintillating material. As shown in FIG. 1, Kwasnick et al. disclose a silicon oxide layer (32) deposited to a thickness of 0.5 to 1.5 microns over the sensor array and a silicon nitride layer (31) deposited to a thickness of 0.05 to 0.15 microns over the silicon oxide layer. This method, however, uses a plasma enhanced chemical vapor deposition process for fabrication of both layers that requires the sensor array to be put back into the deposition chamber.
Furthermore, depending on the material selected for the barrier layer, the corrosive effects of scintillating material may damage the barrier layer as well as the underlying sensor array. Silicon oxynitride is an ideal barrier layer because it provides good optical coupling between the scintillating material and the underlying sensor array. Cesium iodide, however, chemically attacks silicon oxynitride causing metal corrosion in the underlying arrays resulting in severe leakage current and failure of the x-ray detector.
In light of the foregoing, there is a need for a method to protect the barrier layer and the sensor array of an x-ray detector from the corrosive effects of cesium iodide.
Accordingly, the present invention is directed to an x-ray imager including an a-Si sensor array incorporating photodiodes, a barrier layer formed over the sensor array, a benzo-cyclo-butene layer formed over the barrier layer, and an x-ray scintillating material formed over the benzo-cyclo-butene coating.
In another aspect, the invention is directed to a method of making an x-ray imager having an a-Si sensor array protected from the corrosive effects of an x-ray scintillating material including the steps of depositing a barrier layer over the a-Si sensor array, depositing a benzo-cyclo-butene layer over the barrier layer, curing the benzo-cyclo-butene layer in an oxygen free atmosphere, depositing an x-ray scintillating material over the benzo-cyclo-butene layer, and annealing at a temperature up to about 300xc2x0 C.
In another aspect, the invention is directed to a sensor array having a corrosion resistant coating, comprising photosensitive elements, a barrier layer formed over the sensor array, and a benzo-cyclo-butene layer formed over the barrier layer, wherein the benzo-cyclo-butene layer provides corrosion resistance to the sensor array and the barrier layer from a subsequently applied scintillating material.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description serve to explain the principles of the invention.